1. Field of the Invention
The present invention relates generally to active fibers for use as optical amplifiers and lasers for applications ranging from laser-machining and graphic arts to telecommunications, and in particular to 3-level double-clad fiber lasers and 3-level double-clad fiber amplifiers.
2. Technical Background
Optical fiber is increasingly becoming the favored transmission medium for telecommunications due to its high capacity and immunity to electrical noise. Silica optical fiber is relatively inexpensive, and when fabricated as a single transverse mode fiber can transmit signals in the 1550 nm band for many kilometers without amplification or regeneration. However, a need still exists for optical amplification in many fiber networks, either because of the great transmission distances involved, or the optical signal being split into many paths.
As illustrated schematically in FIG. 1, a conventional amplifier 10 is interposed between an input transmission fiber 12 and an output transmission fiber 14. Erbium-doped fiber amplifiers (EDFAs) have been found quite effective in providing the required optical gain, as one example of the amplifier 10. Another example of the amplifier 10 is a fiber with Raman gain. Both transmission fibers 12, 14 need to be single-mode, because higher-order modes exhibit much greater dispersion (typically the limiting factor for the fiber transmission distance at high data rates). The EDFA 10 includes a length (on the order of tens of meters) of an erbium-doped silica fiber 16, as is well known in the art. It is well known that an erbium optical fiber amplifier operating in its purely three-level mode is capable, when pumped at a wavelength of 980 nanometers (nm) of amplifying optical signals having a wavelength of 1550 nm. The doped fiber 16 should also be single-mode in order to maintain the transmission signal integrity. The doped fiber 16 is optically active due to the presence of Er3+ ions or other rare-earth metals, which can be excited to higher electronic energy levels when the doped fiber 16 is pumped by a strong optical pump signal. Typically, an optical pump source 18 inputs the pump signal into the doped fiber 16 through a pump source fiber 20 coupled to either the undoped upstream fiber 12 or the doped fiber 16 through a wavelength-selective directional coupler 22, but downstream coupling is also known. Again, for integrity of the transmission signal, the pump source fiber 20 should be single-mode. An operative EDFA may contain some additional elements (such as an isolator), which are well known to the art but not relevant to the understanding of the background of the present invention.
Conventionally, one typical pump source 18 has been an edge-emitting semiconductor laser that includes a waveguide structure (in what is called a xe2x80x9cstripexe2x80x9d structure) that can be aligned with the single-mode pump source fiber 20 to provide effective power coupling. However, this approach has failed to keep up with modem fiber transmission systems incorporating wavelength-division multiplexing (WDM). In one approach to WDM, a number of independent lasers inject separately modulated optical carrier signals of slightly different wavelengths into the transmission fiber 12. The EDFA has sufficient bandwidth to amplify carrier signals within about a 40 nm bandwidth. A large number of multiplexed signals to be amplified require in aggregate a proportionately large amount of pump power. Over the past decade, the number of WDM channels preferably utilized in a standard network has increased from about four to current levels of forty or more, but at best the output power from a single-stripe laser source has only doubled. Derivative designs such as a master oscillator power amplifier (a single-mode stripe followed by a broad stripe amplifier) or flared-semiconductor devices are capable of producing more than one watt of optical output power, but many of these designs have been subject to reliability problems (such as back-facet damage caused by feedback) that have hindered their practical deployment as fiber amplifier pumps.
Another approach uses WDM technology to combine pump signals. Multiple single-stripe lasers are designed to emit light at narrowly spaced wavelengths, usually within the wavelength bands of 970-990 nm or 1460-1500 nm. Wavelength-dependent directional couplers combine these multiple optical waves into a single (somewhat broadband) pump signal. While this approach increases the power available for optical amplifiers, it greatly adds to the complexity of the pump source, and requires additional components such as thermoelectric coolers, fiber gratings, and directional couplers. As a result, this approach increases cost. At present, the most advanced amplifiers designed for dense wavelength-division multiplexing (DWDM) can use up to six xcx9c150 mW single-mode diode laser pumps. Replacing these six pumps with one broad-area laser can greatly simplify the amplifier design and bring a significant cost advantage.
The single-stripe broad-area diode laser remains the most efficient and least expensive pump source. Recent progress in semiconductor laser technology has led to creation of a single-stripe broad-area laser diodes with output powers of up to 16 W. Devices 100 xcexcm wide with a slow-axis numerical aperture (NA) of less than 0.1 and output power of 2 Watts at 920 and 980 nm are now passing qualification testing for telecommunication applications. With proper coupling optics, the beam of such a laser diode can be focused into a spot as small as 30xc3x975 xcexcm with an NA of less than 0.35 in both transverse directions. The optical power density in such a spot is xcx9c1.3 MW/cm2, which should be high enough to achieve transparency in 3-level laser systems.
One approach for utilizing inexpensive high-power broad-area pump lasers involves cladding-pumped, or double-clad fiber designs. The advantages of cladding-pumped fiber lasers and amplifiers are well known. Such a device effectively serves as a brightness converter, converting a significant part of the multi-mode pump light into a single-mode output at a longer wavelength.
Cladding pumping can be used in a fiber amplifier itself, or employed to build a separate high-power single mode fiber pump laser. A source based on the pure three-level 978 nm Yb+3 transition has long been suggested as a pump for EDFAs because this wavelength is close to the desired pumping wavelength of 980 nm. However, the cladding-pumped technique has been determined in practice to be ineffective for pumping pure three-level fiber lasers, such as the 980 nm transition of ytterbium.
Practical double-clad amplifiers and lasers have been mostly limited to 4-level systems. Double-clad fiber lasers offer better performance for four-level lasing (where the lasing occurs in a transition between two excited states) than for three-level one (where the lasing transition is between the excited and the ground state). For example, for the rare-earth element Ytterbium (Yb) the three-level transition is at 978 nm and competing higher-gain four-level transition is at about 1030-1100 nm.
In a double-clad laser, an outer cladding confines the pump light from a primary pump source in a large cross-sectional area multi-mode inner cladding. The much smaller cross-sectional area core is typically doped with at least one rare-earth element, for example, neodymium or ytterbium, to provide lasing capability in a single-mode output signal. Typically, a neodymium- or ytterbium-doped double-clad fiber is pumped with one or several high-power broad-area diode lasers (at 800 nm or 915 nm) to produce a single transverse mode output (at the neodymium four-level transition of 1060 nm or the ytterbium four level transition of 1030-1120 nm, respectively). Thus, conventional double-clad arrangements facilitate pumping of the fiber using a multi-mode first cladding for accepting and transferring pump energy to a core along the length of the device. Double-clad laser output can be used to pump a cascaded Raman laser to convert the wavelength to around 1480 nm, which is suitable for pumping erbium. To date, a double-clad design by itself (that is, without an additional Raman converter) does not produce a sufficiently high output in any of the appropriate absorption bands for EDFAs.
How much pump light can be coupled into a double-clad fiber inner cladding depends on the cladding size and NA. As is known, the xe2x80x9cetenduexe2x80x9d (numerical aperture multiplied by the aperture dimension or spot size) of the fiber should be equal to or greater than the etendue of the pump source for efficient coupling. The numerical aperture and spot size are different in both axes so there is an etendue in the x and y directions that must be maintained or exceeded.
Typically, a high numerical aperture NAclad, related to the difference in refractive index between the first and second cladding, is desired. In the well-known design, the first clad layer is made of glass and the second is made of plastic (fluorinated polymer) with relatively low refractive index in order to increase the numerical aperture NAclad. Such plastic may not have the desired thermal stability for many applications, may delaminate from the first cladding, and may be susceptible to moisture damage. Furthermore, the known large-cladding double clad concept is not efficient with three-level transitions, such as the 978 nm transition of ytterbium.
An optically active fiber is used for making a fiber laser or an amplifier. This double-clad structured active fiber has a core doped with an optically excitable ion having a three-level transition. The core has a core refractive index and a core cross-sectional area. An inner cladding surrounds the core. The inner cladding has an inner cladding refractive index less than the core refractive index, an inner cladding cross-sectional area between 2 and 25 times greater than that of the core cross-sectional area, and an aspect ratio greater than 1.5:1. An outer cladding surrounds the inner cladding and has an outer cladding refractive index less than the inner cladding refractive index.
The fiber laser or amplifier includes an optical fiber having a core doped with an ion such as erbium (Er3+), neodymium (Nd3+) or ytterbium (Yb3+), which can be optically pumped.
The active fiber and a single-mode output fiber are both designed to provide equal mode field diameters for the lowest-order mode at the junction.
The inner cladding cross-section has an elongated shape with an aspect ratio matching that of the pumping diode laser.
The invention is particularly advantageous when used as a pump source for an erbium-doped fiber amplifier (EDFA) or for the EDFA itself, such as may be found in single-mode fiber optic communication systems or networks.